Hand tools called plasterer's trowls and plows have long been available for smoothing and shaping wet plaster in the corners defined by the adjacent walls of a room. See U.S. Pat. No. 3,079,622, issued Mar. 5, 1963. Such tools are not successfully adaptable to the more currently used dry wall installation approach. A present need is the provision of a trimming tool which will blend the wet filling plaster and the corner ceiling tapes now employed. Such is needed to finish smoothly the corners of dry constructed building walls formed of rigid but shapable wall board with a minimum of manual labor so as to effect an esthetically pleasing seam as to adjoining wall boards. One approach to this construction need is described in U.S. Pat. No. 2,616,285 to S. T. Shields, issued Nov. 4, 1952.